High voltage d.c. converter cathode supply circuit having means for controlling the voltage to the cathode



2 1 v j- R J- UKU Ir-rumor. Qflfinuu uww y 6, 1969 J. W. ACKLEY3,442,252

HIGH VOLTAGE D.C. CONVERTER CATHODE SUPPLY CIRCUIT HAVING MEANS FORCONTROLLING THE VOLTAGE TO THE CATHODE Filed July 22, 1965 INVENTOR;

ATTORNEY United States Patent U.S. c1. i1s--s0.1 12 Claims ABSTRACT OFTHE DISCLOSURE A first rectifier connected to a high voltage transformersupplies current to a load. A second rectifier in parallel with thefirst one is connected to a large capacitor which absorbs the energyproduced by the transformerreactance following an arc in the load, thuspreventing the appearance of a high voltage spike across the load. Aresistor across the capacitor dissipates the energy stored therein.

The present invention relates generally to high voltage D.C. convertersadapted for higher power uses in a vacuum and more particularly to aconverter employing an energy absorbing reactance that is isolatedfromthe high voltage load.

Recently vacuum vapor deposition by electron bombardment heating of anevaporant has been receiving increasingly greater attention because ofrequirements for extremely pure deposited films and layers. Also,electron beam heating affords the only practical method of'evaporatinginany of the refractory material's, e.g., tungsten, because of theextreme temperatures required for vaporization.

To evaporate materials so they can be deposited on a substrate at a highrate by electron bombardment, it is necessary to utilize a higher powerelectron beam, frequently having 2,000 watts power or more at apotential of 4 kilovolts or more. An electron beam of such power, evenin a vacuum less than 4x10" mm. of Hg, sometimes arcs between theelectron source and the'evaporant when certain materials, e.g., quartz,are being deposited. The arcs are usually of short duration, between 10and 1,000 rnicroseconds, and constitute momentary. short circuitsBetween the high voltage terminal at the electron emitting cathode andthe ground potential at which the material being evaporated ismaintained. In orderito extinguish these arcs in the fastest and mostfacile manner, the high voltage power supply for the cathode must besoft, i.e., be capable of delivering only a limited, relatively lowshort circuit currentfor the time period during which arcing takesplace. If the power supply is not soft, but can deliver high shortcircuit currents, arcing can continue for prolonged time periods,whereby the power supply is possibly destroyed or the evaporant maybecome contaminated. For these reasons, prior art high voltagepowersupplies for electron beam vacuum vapor deposition have generallyemployed high voltage transformers with considerable series inductanceto make them current limiting.

A problem arising with the use of transformers having large seriesinductance values is in the extremely high voltages that occur inresponse to extinction of the are. When the arc is extinguished, thetransformer current drops very suddenly and its magnetic field collapsesto provide valves of Patented. May 6, 1969 "ice this approach issatisfactory for certain high voltage.

vacuum applications, it has not generally proven satisfactory for thehigh power cathode rays necessary for elec- T tron beam vapordeposition. Connecting a filter capacitor across the rectifier outputhas been found, through experimentation, to cause repeated additionalarcs to occur after the first arc has been extinguished. Apparently, theheavy discharge currents that flow from the capacitor to the highvoltage terminal in response to the high voltage spikes cause localheating about the cathode. The cathode local heating is sufiicientlyintense to cause gas to be released into the vacuum chamber, whereby amomentary local decrease in the vacuum occurs, enabling an additionalarc to be struck between the high voltage terminal and ground.

According to the present invention, a soft, high voltage power supplyparticularly adapted for electron beam vapor deposition is provided byconnecting an auxiliary rectifier in parallel with the rectifier thatsupplies current to the high voltage output terminal. Connected acrossthe output of the auxiliary rectifier is an energy absorbing capacitor.In response to arc extinction, the capacitor absorbs energy from thecurrent limited power supply to prevent the power supply voltage frombecoming excessive. Since the absorbing capacitor is isolated from thehigh voltage output terminal by the rectifier, its discharge current isnot coupled to the high voltage terminal.

It has been found through tests conducted that repeated arcing issubstantially eliminated by employing the isolated capacitor of thepresent invention when evaporating virtually all materials in vacuums of4 X 10- mm. of Hg.

A further feature of the present invention resides in the use of allsolid state components. The rectifiersemploy high voltage semiconductordiodes,that reduce space and input power requirements. The energyabsorbing capacitor prevents the large over-voltage, discussed supra,from occurring, whereby presently available semiconductor diodes can beused. If the isolated energy absorbing capacitor is not employed, i.e.,the main rectifier operates unfiltered, the peak voltage rating ofpresent-day, relatively inexpensive solid state diodes is exceeded inresponse to the derivation of high 7 voltage spikes from thetransformer.

Another feature of the invention is that the total power supplied by theelectron beam to the evaporant is varied by a single control parameter,the current supplied to the filament of the electron gun. The potentialdelivered to the high voltage output terminal by the supply remainssulficiently constant, at the same value of approximately 4,000 volts,over the 0% ampere range of the beam cur rent, to enable a single knobthat varies beam current to be utilized as the only control required tovary deposition rates.

It is, therefore, an object of the present invention to provide a newand improved A.C to high voltage D.C. converter.

Another object of the invention is to provide a new and improved soft.high voltage power supply, particularly adapted for use in producingelectron beams necessary for vacuum vapor deposition.

An additional object of the invention is to provide a soft high voltagepower supply adapted for deriving relativelyhigh power electron beamsused in vacuum vapor deposition and characterized by its arc suppressingqualities- Yet another object of the invention is to provide a set highvoltage DC. power supply for use in electron beam vacuum vapordeposition systems, wherein the energy from the high voltage spike thatoccurs in response to are extinction is absorbed in a manner thatprevents recurring arcs from being formed.

Still an additional object of the invention is to provide a new andimproved sof high voltage DC. power supply having only solid stateelements so that size and power requirements are minimized.

Yet a further object of the invention is to provide a new and improvedsoft high voltage DC. power supply for use in electron beam vacuum vapordeposition systems, wherein .the'potential from the high voltage. spikethat occurs in response to arc extinction is' attenuated sufficiently toenable semiconductor rectifying diodes to be employed.

Another object of the present invention is to provide a new and improvedhigh voltage power supply adapted for use in deriving high powerelectron beams, wherein the high voltage is regulated suffiiently toenable beam current control to be maintained with a single knob.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawings,wherein:

The single figure is a schematic diagram of a preferred embodiment ofthe invention.

Referring now specifically to the single figure, there is illustratedvacuum vapor deposition chamber 11, maintained during operation at avacuum of no less than 4X l mm. of Hg by a vacuum pump, not shown.Within chamber-11, is electrically grounded metal crucible 12 thatserves as a target electrode for an electron beam 14 that is derivedfrom electron gun 15. Within crucible 12, there is contained evaporantmaterial 13 that isvaporized in response to the kinetic energy ofelectron beam 14. impinging thereon. Electron beam 14 is focused; byconventional means, not shown, to heat material 13- until a puddle isformed. From the puddle, there is derived very pure vapor that driftsupwardly to coat substrate 16, in a a manner well known to those skilledin the art.-

Cathode 15 is maintained at a DC. potential of approximately -4,000volts by the power supply that comprises the present invention and isconnected via a suitable feedthrough into chamber 11. The high voltageDC. potential is derived from a suitable, unregulated AC. low voltagesource, such as a 230 volt, single phase, 60-cycle A.C power supply,applied to terminals 21. The AC. voltage across terminals-21 is applied,in parallel, to variable auto-transformer 22 and primary winding 23 oftransformer 24. Winding 23 is coupled through core 25 to secondarywinding 26, across.which is generated an A.C. voltage of approximately4.000 volts RMS'. 1

Core 25 is such that transformer 24 is characterized as having arelatively large inductance, whereby the maxi mum short circuit currentderived is approximately 2 amperesJHence, transformer 24 is consideredas a. soft supply.

The high voltage A.C. across secondary 26 is applied in parallel to fullwave rectifying bridges 27 and 28. Each of bridges 27 and 28 includesfour semiconductor diodes 29 poled so that terminals 31 are maintained;approxi mately 4,000 volts negative with respect to terminals 32thereof. Connected in parallel across output terminals 31 and 32 ofbridge 28 is a two microfarad, 10,000-volt energy absorbing capacitor 33and 700,000 ohm, 20-watt bleeder resistor 34. Terminals 31 and 3-2 ofbridge 27 are con= nected unfiltered between ground and cathode 15through 4 1 electron current monitoring rnilliammeter 3-5, having a fullscale deflection of 500 milliamps.

Heating current, of up to 25 amperes at 6 volts AC. is supplied acrosscathode 15 from the tap on auto-transformer 22 through stepdowntransformer 36. One end of secondary winding 37 of transformer 36 isconnected to terminal-=31 of bridge 27. As the setting of the tap onauto-transformer 22 is varied, the heating current to cathode 15 ischanged, whereby the current in electron beam 14 is altered, as is therate of vaporization from material 13 in crucible 12. The power supplyhas sufficient self-regulation to maintain terminal 31 ata relativelyconstant DC potential of -4,000 volts for all values of beam currentfrom 0 to 0.5 ampere. Therefore, 7

it has been found that the tap on transformer 22 is the only controlrequired to provide the full range of beam current values necessary tovaporize material 13 for many different deposition rates. In normal:operation, electron beam current flows from cathode 15 to its target,evaporant 13. Electron beam current flow varies from virtually 0 ampereto its maximum value, as determined by the setting of the tap fortransformer 22 during each half cycle of the AC. source because of theunfiltered nature of the supply.

As described supra, this causes heating of evaporant and deposition onsubstrate 16. Occasionally, and for many different reasons, arcs arestruck' betweent cathode 15 and evaporant 13. These arcs are of shortduration, lasting between 10 and 1,000 microseconds, and are limited totwo amperes because transformer 24 comprises a soft supply. The voltagebetween cathode 15 and target 13 drops almost to zero during theoccurrence of an arc. The low cathode voltage does not enable the are tobe maintained for prolonged time periods, so extinction occurs withinthe stated interval.

When each arc is extinguished, there is considerable energy stored in,core 25. Because arc extinction is very rapid, the stored energy has atendency to induce a very large voltage spike across transformersecondary 26. The spike, regardless of polarity, is rectified by bridge28 and'absorbed by the very low impedance of capacitor 33 to its largeamplitude, high frequency components. As the voltage across secondary 26returns to its normal quiescent value, the charge stored by capacitor 33leaks through bleeder resistor 34 so that the capacitor can suppressanother spike. Because capacitor 33 cannot discharge through bridge 28,due to the polarity of diodes 29 in the bridge, there is no currentsurge at cathode 15 shorty after arc extinction and a further arc cannotbe triggered. Capacitor 33 is sufficiently large to prevent the voltagespike across winding 26 from ever exceeding 6,000 volts so that presentstate of the art semiconductor diodes, such as type l44 6-C, availablefrom Diodes, Incorporated, can be employed.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variations of the details ofconstruction which are specifically illustrated'and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

I claim: I

1. A system for coverting power from an A.C. source to high voltage DC.to be delivered to' a load comprising a transformer having: a primarywinding adapted to be connected to said source, secondary winding meansacross which is derived high voltage AC, and a core; first and secondrectifiers connected in parallel across said second- 3. The system ofclaim 2 wherein said rectifiers are full wave bridge rectifiers.

4. A system for converting power from an AC. source to high voltage tobe delivered to a load comprising a transformer having: a primarywinding adapted to be connected to said source, secondary winding meansacross which is derived high voltage A.C., and a core, a rectifierconnected across said secondary winding means .for connecting highvoltage deriving from said secondary winding means to said load, and anenergy absorbing reactance connected across the output of saidrectifier, said rectifier being poled to couple energy spikes from thetransformer to said reactance and to prevent discharge of energy fromsaid reactance to said load.

5. A system for converting power from an AC. source to high voltage forbiasing an electron gun that emits a high powered electron beam directedtoward a target within a vacuum chamber comprising a transformer having:a primary winding adapted to be connected to said power source,secondary windings means across which is derived high voltage AC, and acore, said transformer limiting electron beam current in the event ofarcing between the cathode and the target and in which energy is storedduring said arcing; means for coupling high voltage deriving from saidsecondary winding means to said cathode so unfiltered electron beamcurrent flows from the cathode to the target during at least every otherhalf cycle of said A.C. supply, rectifying means connected across saidsecondary winding means, said rectifying means being separatelyresponsive to the voltage across said secondary winding means from thevoltage coupled by said secondary winding means to said cathode, acapacitor connected across said rectifying means for ab sorbing energyderiving from said transformer in response to are extinction, saidrectifying means being poled to coupled energy from the transformer tosaid capacitor and to prevent energy stored in said capacitor from beingcoupled to said cathode.

6. A system for converting power from an A.C. source to high voltage forbiasing an electron gun that emits a high powered electron beam directedtowarda target within a vacuum chamber comprising a transformer having:a primary winding adapted to be connected to said power source,secondary Winding means across which is derived high voltage AC, and acore, said transformer limiting electron beam current in the event ofarcing between the cathode and the target and in which energy is storedduring said arcing; means for coupling high voltage deriving from saidsecondary winding means to said cathode so unfiltered electron beamcurrent flows from the cathode to the target during at least every otherhalf cycle of said A.C. supply, a reactance for absorbing energyderiving from said transformer in response to are extinction, and meansresponsive to the high voltage A.C. across said secondary winding forcoupling energy deriving from the transformer as a result of arcextinction to the reactance and preventing coupling of energy from thereactance to the cathode, said last-named means being separatelyresponsive to the high voltage A.C. across said secondary winding fromthe voltage coupled to said cathode.

7. A system for converting power from an AC. source to high voltage D.C.for biasing an electron gun that emits a high powered electron beamdirected toward a target within a vacuum chamber comprising atransformer having: a primary winding adapted to be connected to saidpower source, secondary winding means across which is derived highvoltage A.C., and a core, said transformer limiting electron beamcurrent in the event of arcing between the cathode and the target and inwhich energy is stored during said arcing; first rectifying means c0nnected across said secondary means for deriving a high voltageunfiltered rectified replica of the AC. voltage source, means forsupplying said replica to the cathode, second rectifying meansresponsive to the high voltage A.C. across said secondary winding means,said second rectifying means being responsive to the high voltage A.'C.across said secondary winding means separately from the voltage coupledto said first rectifying means, a capacitor connected across said secondrectifying means for absorbing energy deriving from said transformer inresponse to are extinction, said second'rectifying means being poled tocouple energy from the transformer to said capacitor and to preventenergy stored in said capacitor from being coupled to said firstrectifying means.

8. The system of claim 7 further including stepdown trans-former meansresponsive to said AC. power source for deriving heating current forsaid cathode, means responsive to said stepdown transformer means forcou-.

pling said heating current to said cathode, said stepdown transformermeans including means for varying the heating current supplied to saidcathode as the only control parameter of the system.

9. The system of claim 8 wherein said first rectifying means comprises afull wave bridge rectifier having: a pair of input terminals connectedacross said secondary winding means and a pair of output terminalsconnected between said cathode and target; and a current measuringmeterfor monitoring the current of the electron beam, said meter beingconnected in series circuit with said pair of output terminals, saidcathode and said target.

10. The system of claim 7 wherein said second rectifying means comprisesa full wave rectifying bridge having a pair of input terminals connectedacross said secondary winding means and a pair of output terminals; saidcapacitor being connected across said output terminals, and a bleederresistance for said capacitor connected in parallel with said capacitor.

11. In combination, a transformer having: a low voltage primary winding,a core, and a high voltage secondary winding; first and second full waverectifier bridges having their inputs connected in parallel across saidsec,- ondary winding, the parallel combination of a resistor andcapacitor connected across the output terminals of said first bridge,and a pair of high voltage output terminals connected to the outputterminals of said second bridgel 12. In combination, a transformerhaving: a low voltage ,primary winding, a core, and a high voltagesecondary winding; first and second full wave rectifier bridges havingtheir inputs connected in parallel across said secondary winding, theparallel combination of a resistor and capacitor connected across theoutput terminals of said first bridge, a vacuum vapor deposition chamberhaving a target adapted to carry an evaporant, a cathode for emitting anelectron beam to heat said evaporant to vaporization, and means forapplying the unfiltered voltage deriving from said second bridge acrosssaid cathode and said target.

References Cited UNITED STATES PATENTS 12/1949 Zavales 250 2/1963 Hankset al. 3l5--107 US. Cl. X.R.

219-12l; 250-495; 31510l, 107, 205; 32l10; 323-8

